This one trips up almost every convert, and it trips them up quietly, which is worse. You are reading a homily. St. John Chrysostom or St. Basil quotes Psalm 50 and says something beautiful about repentance. You go look up Psalm 50 in your Bible so you can read the whole thing. And you get a psalm about God calling the heavens and the earth to witness.
That is not the psalm about repentance. So what happened? Did the saint misquote? Did somebody get the footnote wrong?
No. You just walked into one of the oldest numbering differences in the Bible, and once you understand it you will never be confused by it again.
Two ways of counting
The Psalter in the Septuagint and the Psalter in the Hebrew Masoretic text contain the same material, but they divide it differently in a few places. The biggest split happens early.
In the Hebrew, Psalms 9 and 10 are two separate psalms. In the Greek, they are one psalm. Open Psalm 9 in the Theologians Bible and count. It runs to thirty nine verses, because it contains what your English Bible prints as two psalms.
One merge at the front means everything behind it shifts. From that point on, through most of the Psalter, the Greek number is one lower than the Hebrew number. Your Psalm 23 is Psalm 22. Your Psalm 51 is Psalm 50. Your Psalm 91 is Psalm 90.
So when the saint quoted Psalm 50, he meant this:
For the end, a Psalm of David, when Nathan the prophet came to him, when he had gone to Bersabee. Have mercy upon me, O God, according to your great mercy.
(Psalm 50:1)
Theologians Bible
That is the Miserere. That is the psalm of repentance that Orthodox Christians pray constantly, the psalm David wrote after Nathan confronted him over Bathsheba. In your English Bible it is numbered 51. The saint was not wrong. He was counting the way the Church counts.
The shepherd psalm
The most famous case is the one in the title of this article. Everybody knows Psalm 23. Here is how it opens in the Septuagint:
A Psalm of David. The Lord tends me as a shepherd, and I shall lack nothing.
(Psalm 22:1)
Theologians Bible
Same psalm. Different number. And notice the wording is not identical either, because you are reading a translation of the Greek rather than a translation of the Hebrew. "The Lord tends me as a shepherd" is doing something slightly different than "The Lord is my shepherd." Neither is wrong. They are two faithful renderings of two ancient forms of the same psalm.
There is a psalm you may never have read
Here is something else. Open Psalm 151.
Your Bible almost certainly stops at 150. The Septuagint Psalter carries one more, and the edition marks it plainly as supernumerary, meaning it stands outside the count of the hundred and fifty. It is David's own account of being taken from the sheep and anointed, and of going out against Goliath.
It is short. Go read it. It takes a minute, and most Christians in the English speaking world have never laid eyes on it.
How not to get lost
The practical problem is real. Orthodox service books, patristic homilies, and the Fathers' commentaries all use the Septuagint numbers. Most English study aids, concordances, and cross reference systems use the Hebrew numbers. You are going to be moving between the two constantly.
So the reader carries both. When you open a psalm in the Old Testament, the other tradition's number is printed right there with the text. Psalm 50 tells you it is 51. Psalm 22 tells you it is 23. You do not have to do arithmetic in your head or keep a conversion chart in a drawer.
My rule is simple. When a Father, a service, or an Orthodox book gives you a psalm number, assume it is the Septuagint number and add one to find it in a Protestant Bible. When a Protestant book gives you a number, subtract one to find it in the Fathers. Then confirm by reading the first line, because the first line never lies. In the range where the two systems diverge, this will get you where you are going almost every time.
Why this matters more than a numbering quirk
I could have written this as a two paragraph note. I did not, because the numbering is a symptom of something bigger.
The Psalter is the prayer book of the Church. It is the single most quoted book in the New Testament. It is prayed in every service. And the Psalter the Church has prayed for two thousand years is the Greek one, with its own divisions, its own headings, and in places its own wording.
When you notice that the numbers do not line up, you are noticing that you have been handed a different edition of the Church's prayer book than the one the Church has actually been praying. That is worth knowing. Not because the other one is worthless, but because you should know which one you are holding.
Start here. Open Psalm 50. Read it slowly. Then read what the Fathers say about it. Then pray it. That is the psalm the Church puts in your mouth when you have nothing left to say for yourself, and it has been numbered 50 the entire time.